1. Field of the Invention
The present invention relates generally to CMOS image sensors, and more particularly to using interpolation for color correction in a CMOS image sensor.
2. Background of the Invention
An image sensor is generally classified into a CCD (charge coupled device) image sensor or a CMOS (complementary metal-oxide semiconductor) image sensor (CIS). Compared to the CCD, the CIS is fabricated economically with a general CMOS process and is advantageous for integration of analog/digital signal processing circuits. Also, the CIS which is amenable to low power and low voltage design is widely used in portable devices such as mobile phones or digital cameras.
The CIS includes an array of pixels each being activated with timing control for outputting a respective image signal corresponding to received light. A color filter array (CFA) having a color pattern such as the Bayer color pattern is installed on the CIS such that each pixel detects the intensity of light of a respective color such as red (R), green (G), or blue (B) for example. In particular, the G of the Bayer CFA is one of Gr or Gb with Gr being located between neighboring R and other G pixels and Gb being located between B and other G pixels.
Although each pixel senses light of a respective color, color interpolation or demosaicing algorithms are used to determine the color components R, B, and G (including Gr and Gb) for each pixel of the pixel array. The difference between the Gr and Gb is ideally “0” for each pixel. When the difference between the Gr and Gb color components is not “0”, noise may be generated when displaying the image by as much as the difference between the Gr and Gb color components.
Each pixel of the CIS pixel array uses transistors and a pixel diode such as four MOS transistors for a 4-Tr structure. The photodiode accumulates electric charge corresponding to the intensity of received light. The transistors are used for controlling the timing of transmission of the accumulated electric charge from the photodiode to a floating diffusion node and for amplifying the voltage at the floating diffusion node for generating an output voltage corresponding to the accumulated electric charge.
The pixel size of the CMOS image sensor is gradually decreased according to demand for high resolution. Thus, multiple pixels of the CMOS image sensor may share transistors, the photodiode, or/and the floating diffusion node. However, sharing of such components may result in variance in the output voltages generated by such sharing pixels because of irregular arrangement and layout of such shared components in the sharing pixels. Thus, a defective color tone may result in the image with a difference between the Gr and Gb color components.